The invention relates to a spectrometer for measuring inelastically scattered light. Raman spectrometers are an example of spectrometers for measuring inelastically scattered light. When a sample is irradiated with monochromatic light, the light scattered by the sample will contain wavelength components different from those present in the incident light. So-called Raman scattering of light on molecules present in the sample creates this effect. With a Raman spectrometer, a sample is illuminated with a strong, essentially monochromatic light source. The light which is scattered by the sample is collected and the spectral distribution is analysed for spectral components with a wavelength different from that of the light source.
U.S. Pat. No. 5,862,272 describes the use of optic fibres for guiding Raman spectra of samples to and from the sample, while the sample is at a distance from the light source which is used for excitation and/or at a distance from the equipment for detection of the Raman signal. The use of optic fibres enables measurements at locations which are otherwise difficult to reach or cannot be reached at all, as is the case, for instance, in in vivo measurements on tissue, for instance in blood vessel examination or more generally in endoscopic applications, or as is the case in industrial applications such as in-line process control.
U.S. Pat. No. 5,862,273 describes the use of a filter at the extremity of the fibre (excitation-fibre) which guides the monochromatic light from the light source to the sample, adjacent the sample to be examined. This filter (excitation filter) only transmits light of the wavelength of the light source and no Rams scattered light from the fibre, which is generated therein by inelastic scattering of light from the monochromatic light source. This enables sensitive measurement, without interference by inelastically scattered light from the fibre which returns to the analysis unit after scattering or reflection on the sample.
Further, the use of an excitation filter in or an the excitation fibre necessitates the use of a different fibre or fibres (collection fibres) for collecting light scattered by the sample and its guidance to the analysis unit, since otherwise the Raman scattered light of interest would be stopped by the excitation filter so that it cannot reach the analysis unit. Special measures are necessary to obtain a good overlap between the volume or surface of the sample irradiated by light from the excitation fibre and the volume or surface of the sample from which scattered light is intercepted by the collection-fibre(s). Also, the use of separate fibres for supply of light to and discharge of light from the sample renders Raman probes complex. The signal collection efficiency is suboptimal and far-reaching miniaturisation is impeded.
The light scattered by the sample largely consists of elastically scattered light, having the same wavelength as the incident light from the monochromatic light source. This elastically scattered light of considerable intensity can tin give rise to the generation of Raman signal in the collection fibres, which leads to an interfering signal background. For that reason, for sensitive measurements, a filter (emission filter) is utilized before, on or in the collection fibre(s) on the side where the sample is located, which serves to stop the light scattered elastically by the sample and to transmit only the scattered light which is shifted in wavelength with respect to the wavelength of the monochromatic light source.
Using excitation and emission filters limits the use of the spectrometer to a light source of a particular wavelength and/or measurement of the Raman signal in a particular wavelength range. The necessity of the use of optic filters before, in or on the fibres close to the sample renders fibre optic probes for Raman spectroscopy complex.
Also from U.S. Pat. No. 6,208,887, a Raman spectroscope is known. This patent discusses the possibility of using a multimode laser. The sample is illuminated via an optical fibre, without the use of optic filters, so that the spectrometer is only suitable for low sensitivities. For receiving scattered light the possibility of any light guide (waveguide) is mentioned, without, however, mentioning the advantages thereof over optic fibres.
It is one object of the invention to provide a spectrometer for inelastic scattering with a simplified construction of the probe.
It is another object of the invention to provide such a spectrometer wherein further-reaching miniaturisation of the probe is possible.
It is a further object of the invention, among still further objects, to provide such a spectrometer which is designed such that sensitive measurements are possible, while no filters are required or fewer filters than in the prior art.